Nowadays many types of image generating devices exist, including phototypesetters, imagesetters, lithographic printers and electronic printers for printing electronic (colour) images. The generated images may be e.g. latent, visible or lithographic and are generated on a suitable image carrier. An image carrier may be paper, a transparent PET (polyethyleenterephtalate) material, photographic material, an electrophotographic drum or a lithographic printing plate etc. A non-visible image usually undergoes a process to generate a visible image from it: a latent image may be developed; a lithographic image, comprising ink accepting and ink repellent zones, may be provided with ink, which is transferred to a paper image carrier to render a visible image.
Some printers use thermal processes to form an image. These may be direct thermal systems, thermal (wax) transfer system or thermal systems using dye sublimation to form images on a receiving material or image carrier. The thermal process can be activated by using a thermal head or infrared (IR) light sources. An IR light source commonly used in laser thermal printers is a semiconductor laser. Other popular printing systems use an ink jet printing technology. Droplets of fluid ink are ejected to a receiving layer or image carrier to form a visible image.
A very common type of printer in the office environment is a printer using an electrographic process. According to the electrophotographic process, which is a specific electrographic process, a latent electrostatic image is formed by selectively illuminating or exposing an electrostatically charged photoconductive drum and developing the latent image by toner, thereby producing a visual toner image. The toner may thereafter be transferred to an image carrier or substrate made of e.g. paper.
Another electrographic process is referred to as Direct Electrographic Printing (DEP) and is described in e.g. EP-A-0 675 417. According to this technique, a toner cloud is brought in the vicinity of a print head structure. That structure has apertures that may be “opened or closed” by electrostatic action. By image-wise opening these apertures, toner particles travel image-wise through the apertures of the print head structure and impinge on an image substrate such as an intermediate image drum or a final image substrate such as paper or a transparency material. Most of the above mentioned printers use print heads: these are units carrying the image-forming or image-generating elements, and which e.g. provide the heath, emit light or eject ink or toner particles in an image wise fashion.
A print head is generally not capable to generate at once the complete image on the image carrier. For generating an image, the area of the image carrier is traditionally (mentally) partitioned in tiny addressable units, referred to as microdots. These microdots are disjunctive, i.e. they do not overlap each other, and all the microdots together fill the complete image area on the carrier. As such, they form a real partition of the image carrier. The microdots may be obtained by a grid defined by a first set of parallel equidistant lines having a first orientation and a second set of equidistant parallel lines having a second orientation different from the first orientation. The tiny parallelogram areas, enclosed by two sets of two parallel line portions, are referred to as microdots. If the second orientation is orthogonal to the first orientation, then the microdots have a rectangular shape. If the distance between two consecutive lines of the first set equals to such distance of the second set, then the microdots have a rhombic shape. In most cases the orientation is orthogonal and the distance is identical, resulting in square microdots. The multiplicative inverse of the size of the side of a square microdot is referred to as the spatial resolution of the imaging device. The microdot size in an electrophotographic device may be 42 micron (μm). The spatial resolution of the device is then 1000/42 microdots per mm, i.e. 24 dots per mm or 600 dots per inch (600 dpi). The notion of an “addressable” microdot refers to the fact that the imaging device is capable to address the microdot individually. A binary electrographic device is capable to deposit either a maximum amount of toner or a minimum amount of toner on each individual microdot.
Although the microdots are disjunctive, is may be possible that some toner particles designated for a first microdot, also partly cover an adjacent microdot, i.e. a microdot that has a side or a corner in common with the first microdot. Examples of such binary devices are the Agfa P400, P3400 and P3400PS devices, developed and marketed by Agfa-Gevaert N. V. in Mortsel, Belgium and having a resolution of 400 dpi. A multilevel electrographic device is capable to deposit on each individual microdot specific variable amounts of toner, expressed in microgram per square millimeter (μg/mm2). The number of such specific amounts may be e.g. 16, such as in the Chromapress system, developed and marketed by Agfa-Gevaert N. V. in Mortsel. The lowest amount of toner may be generated by offering to the electrophotographic system a digital value 0, whereas the highest amount of toner may be generated by offering to the electrophotographic system a digital value 15. All values between 0 and 15 may generate on the individually addressed microdot each a specific amount of toner between said minimum and maximum amount. Since values from 0 to 15 may be represented by four bits, this system is referred to as a 4-bit multilevel system. To achieve the impression of continuous tone for images reproduced on such system, it may be necessary to introduce some form of halftoning, as described in EP-A-0 680 195, EP-A-0 634 862 and EP-A-0 682 438.
It follows that an A4-sized image carrier (297 mm×210 mm) comprises about 35 million microdots in a 600 dpi (ca. 24 microdots per mm) system. An imagesetter for generating a printable image may have a spatial resolution of 2400 dpi (ca. 95 microdots per mm). If the film or printing plate has a size of 14″×17″ (14 inch by 17 inch, i.e. 356 mm by 432 mm), the number of microdots on the image carrier amounts to 1,371 million. An imaging device capable to address that large number of microdots at once and at that resolution would be too costly. Therefore, the image carrier is rather exposed line by line, e.g. by using a LED array or even pixel by pixel—i.e. microdot by microdot—by using a sweeping laser beam. A sweeping laser beam may be generated by an imaging device comprising a laser source and a light deflection system such as a rotating polygonal mirror or a rotating pentaprism. In more complex systems, all microdots arranged on a plurality of lines may be addressed at once, i.e. at the same instant. This may be achieved by a plurality of parallel LED arrays in an electrophotographic system and by a print head structure having a plurality of parallel lines of printing apertures in a DEP device. A plurality of sweeping laser beams may give the imaging device the capability to address a plurality of microdots at once. According to the above mentioned systems, one imaging device—i.e. one LED array, one printhead structure, or one laser beam system comprising a laser source and a deflection means—is capable to address one line of microdots simultaneously or at least within a short period of time. With that short period is meant the time to address all the microdots of one line, without addressing within that period other lines by the same imaging device.
Due to the cost of some complex devices, it is sometimes too expensive to provide a print head having a length equal or larger than the width of the recording material. For some technologies it is even impossible to make a good quality print head of a large size. As such, the shorter print head cannot address instantly all microdots arranged on one line running from one side to the opposite side of the image carrier.
Especially when printing a large size image, e.g. posters, the print head can print only a portion of the image. A poster may have a size of 1.5 m width and 2.5 m length. In a 75 dpi system, state of the art systems have a printing head with a width of 30 cm. To cope with a poster width of 150 cm, the printing head has to make at least five steps. Therefore the image is printed in several parallel bands, referred to as sub-images, which are sequentially printed alongside each other.
In an inkjet printer having an array of nozzles arranged in a longitudinal direction parallel to the longer side of the paper to be printed, the paper is fed stepwise relative to the print head in a longitudinal direction. The print head has a transversal shuttle movement relative to the paper for printing image bands by simultaneous operation of the plurality of nozzles. The bands are printed one after another. A first image band or sub-image is printed during a first transversal shuttle movement. Thereafter, the paper is moved stepwise in a first longitudinal movement. Then the second sub-image is printed during a second transversal shuttle movement, followed by a second longitudinal stepwise movement etc.
In a thermal laser transfer printer the imaging material can be mounted on a drum. While the drum is rotated the print head is stepwise moved along the rotation axis printing the image band sequentially alongside each other. One such printer is described in WO 93/04 552 where a thermal print head carrying diode lasers coupled to fibres is displaced alongside the rotatable drum.
Sequentially printing the image in bands or sub-images may give the following problems.    1. When image bands do not exactly join together it is possible that a distinct white line in between the printed bands becomes clearly visible as an image defect. On the other hand, when the image bands overlap, a clearly visible dark line disturbs the image.    2. Even when print bands join perfectly along the length, a slight mismatch in the position of the bands along this length can cause visible artefacts.
As can be seen in FIG. 2, the mismatch due to displacement of a first sub-image 21 relative to a second sub-image 22 according to the size of only one microdot can cause a visible defect when printing screened (binary) images. This is referred to as a phase defect of the screened data.
The artefacts, caused in the image zone 26 where two sub-images join, may find their origin in the imperfect placement of the printing head for printing the second sub-image in relation to the first printed sub-image. This may be due to play of the mounting and moving system of the printing head.
The very accurate positioning systems, needed to solve the above problem, are too expensive to install in the printers destined to the consumer market.
The above problems have already been recognised by other researchers and several solutions have been proposed.
In EP-A-0 522 980 and EP-A-0 619 188 there is proposed to make an overlap zone of two printed bands in a thermal sublimation printer where two fitting stochastic rasters gradually fade towards the neighbouring band.
In a laser thermal transfer proofer described in EP-A-0 529 535 and WO 93/4 552 the outermost lines of each band are so called “dummy” lines. The information recorded in these lines has the purpose to avoid the occurrence of white side lines due to incorrect placement.
In DE-A-4 110 776 the joining of the bands in an ink jet printer using a shuttling print head is not done along a straight line but along a curved (random) path.
Despite of all the proposed measurements hitherto, there is still a need to obtain a good quality joining of printed bands.